Changed git clone strategy for CI tests.

Previously, it seems like build artifacts from previous runs (such as
.sch files and logs) were retained.  We need to make sure we have a
clean build.

.gitlab-ci.yml

@@ -10,6 +10,8 @@ before_script:
 
 build_libmesh_job:
   stage: build_libmesh
+  variables:
+    GIT_STRATEGY: clone
   script:
     - pwd
     - git clone -b master https://github.com/idaholab/moose.git